Synchronous rectification DC-DC converters are known as power supply devices using synchronous rectification. The synchronous rectification DC-DC converter uses a FET (field effect transistor), and switches between an on-state and an off-state of a gate of the FET at a necessary timing in synchronization with a clock, thus performing operation of the synchronous rectification. Some synchronous rectification DC-DC converters have a backflow prevention circuit for preventing backflow. The backflow prevention circuit detects a voltage of a switching terminal immediately after the FET configured to perform synchronous rectification operation is turned off, thus determining whether backflow is occurring. Additionally, if it is determined that the backflow is occurring, the backflow prevention circuit prevents the FET from entering the on-state, thus preventing backflow (see, for example, Patent Document 1).
Additionally, in a case where a synchronous rectification DC-DC converter is mounted on a vehicle or the like, there are some cases where an external power source, such as a battery, is connected to an output terminal of the synchronous rectification DC-DC converter. Further, there are some synchronous rectification DC-DC converters whose output is connected to a choke coil. Additionally, when a battery is connected to an output terminal, there are some cases where a voltage of the battery becomes higher than an output voltage of the synchronous rectification DC-DC converter.